Versatile self-assembled electrospun micropyramid arrays for high-performance on-skin devices with minimal sensory interference

On-skin devices that show both high performance and imperceptibility are desired for physiological information detection, individual protection, and bioenergy conversion with minimal sensory interference. Herein, versatile electrospun micropyramid arrays (EMPAs) combined with ultrathin, ultralight, gas-permeable structures are developed through a self-assembly technology based on wet heterostructured electrified jets to endow various on-skin devices with both superior performance and imperceptibility. The designable self-assembly allows structural and material optimization of EMPAs for on-skin devices applied in daytime radiative cooling, pressure sensing, and bioenergy harvesting. A temperature drop of ~4 °C is obtained via an EMPA-based radiative cooling fabric under a solar intensity of 1 kW m–2. Moreover, detection of an ultraweak fingertip pulse for health diagnosis during monitoring of natural finger manipulation over a wide frequency range is realized by an EMPA piezocapacitive-triboelectric hybrid sensor, which has high sensitivity (19 kPa−1), ultralow detection limit (0.05 Pa), and ultrafast response (≤0.8 ms). Additionally, EMPA nanogenerators with high triboelectric and piezoelectric outputs achieve reliable biomechanical energy harvesting. The flexible self-assembly of EMPAs exhibits immense potential in superb individual healthcare and excellent human-machine interaction in an interference-free and comfortable manner.


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Co., Ltd., China) and heater were respectively used to control the humidity and temperature during electrospinning. A smooth aluminum foil was wrapped around the drum to collect electrospun fibers. The arithmetical mean height, maximum peak height, and maximum pit height of the aluminum foil are 21.9, 208, and 209 nm, respectively. After electrospinning, asprepared samples were dried at room temperature until solvent completely volatilized to obtain easy-to-release electrospun films containing 3D micropyramid arrays or 2D planes.
More detailed electrospinning parameters for fabricating various electrospun films are presented in Supplementary Tab. 2.

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Fabrication of electrospun-film-based piezocapacitive sensors. The electrospun-film-based piezocapacitive sensor was composed of two layers, a top Au flat electrospun film and a top Au electrospun micropyramid-arrayed film, which clung to each other ( Supplementary Fig.   17a). For comparison, the reference sample comprised two top Au flat electrospun films. The Au was deposited using magnetron sputtering (K550X, Emitech, England). The working area for all piezocapacitive sensors was 10 × 10 mm 2 . The piezocapacitive sensor was stuck to a glass substrate and covered by a smooth protective layer for the sake of relative capacitance change versus pressure curve test.
Fabrication of on-skin piezocapacitive sensors. In terms of EMPA-based piezocapacitive sensor, the PVA nanofibers were deposited onto a fingertip by electrospinning and were then wetted by water mist to firmly stick the sensor to the fingertip ( Fig. 3c and Supplementary Fig.   14f). The non-working area between the flat electrospun film and the EMPAs was also treated through wet electrospun PVA nanofibers to guarantee fine contact between two layers of the sensor. Body temperature can soon dry out gas-permeable PVA glue within 3 min.
Subsequently, the on-skin EMPA-based sensor started to record fingertip pulses and pressure signals (Fig. 5d, f). Similarly, a top Au flat polypropylene film and a top Au micropyramidarrayed PDMS film made up the conventional PDMS micropyramid-array-based piezocapacitive sensor ( Supplementary Fig. 14e) 1 . Due to the unsatisfactory flexibility, large thickness and heavy weight of this conventional sensor, ordinary glues cannot stick it to the finger. Therefore, this conventional sensor was fixed on the fingertip by wrapping a scotch tape ( Supplementary Fig. 14e). The working area of the two types of aforementioned on-skin sensors was approximately 10 × 10 mm 2 .  Fig. 23 and Fig. 5f). These two electrospun films (i.e., a pair of tribolayers) are in contact to monitor contact and separation states during clicking mouse. In the biomechanical energy harvesting experiment, the working area of the TENG consisting of a top Au 3D EMPA-M and a top Au 2D EP was 10 × 10 mm 2 ( Supplementary   Fig. 18b). It is noteworthy that all PVDF samples were depolarized by annealing at 150°C for 24 h to avoid interference from piezoelectric effect 2 and the annealing procedure exerted no influence on sample morphology ( Supplementary Fig. 18c).
Fabrication of electrospun-film-based PENGs. The electrospun piezoelectric PVDF film was sandwiched between two Au electrodes, and then the resultant device was encapsulated using PI tapes. Meanwhile, the test surface of the cycle impact apparatus was also covered by a piece of PI film to minimize the interference from triboelectric charges 3 ( Supplementary Fig.   21c). The working area for all PENGs was 20 × 20 mm 2 .
Local electric field simulation. All models were built by SolidWorks 2016 software. The model of the electrospinning machine consisted of a box full of air (650 × 600 × 450 mm 3 ), a grounded aluminum collector (length 300 mm), and a steel spinneret (external diameter 0.71 mm, internal diameter 0.41 mm, length 28 mm). The distance from the spinneret to the collector was 80 mm. The applied voltage was +12.5 kV. The models of the electrospun microdome and micropyramid were 5.12 and 26.1 μm in height, respectively. Subsequently, the models were imported into COMSOL Multiphysics 6.0 with an AC/DC module for S7 electrostatic field simulation. Preference was given to the software material library: air (Air), aluminum (UNS A91050), and steel (UNS S31600).
Characterization of breathability. Deionized water of 1 g was placed in a glass bottle with a height of 98 mm and a diameter of 15 mm at the opening, and then the sample was attached to the opening of the bottle (Supplementary Fig. 14a). Subsequently, a glass cover was used to avoid the interference from strong winds in the constant climate chamber ( Supplementary Fig.   14b). Additionally, there was a 32 mm gap between the glass cover and the turntable, which made the temperature and humidity inside and outside the glass cover consistent. The turntable with a rotation speed of 3 rpm was used to make any possible interference towards each of the test samples equal ( Supplementary Fig. 14b). Finally, the installation was stored in the constant climate chamber at 25°C and a humidity of 30% ( Supplementary Fig. 14c).
Meanwhile, the subsequent decrease in weight was tested.
Human research participants. Twenty adults aged 21-31 participated in the biocompatibility study, object-grasping experiment, and fingertip pulse waveform longduration monitoring.
Biocompatibility study and fingertip pulse waveform long-duration monitoring. Twenty adults aged 21-31 took part in this study. The electrospun and conventional PDMS micropyramidarray-based piezocapacitive sensors were attached to fingertips for 7 h. Participants reported any feelings during the duration of the tests for each of the two sensors. The VAS (0-10) was used for this report, and the details are illustrated in Supplementary Tab. 3. One healthy male aged 27 took part in the fingertip pulse waveform long-duration monitoring. The questionnaire survey was conducted anonymously.
Object-grasping experiment. Eighteen right-handed participants aged 21-30 with no reported neurological disorders took part in this measurement. Participants were seated in front of an instrumented object which rested on an opaque box to avoid the visual influence. A multidimensional force sensor (Shenzhen Ruilide Technology Co., Ltd., China) embedded in the S8 object was used to record the simultaneous load force and grip force between the thumb and index fingers. The sensor-equipped object weighed 58 g (corresponding to a load force of 0.57 N). Different loads (50, 100 or 150 g) could then be suspended from the bottom of the object to change the load force (1.06, 1.55 or 2.04 N). The recorded load force of the object was programmatically corrected before the test. The monitor rendered acoustic control of the timing of the task. Each participant lifted the object 3 cm above the box and hold it for 5 s by thumb and index fingers. Subsequently, the object was placed down. Three finger surface conditions (bare finger, EMPA device, and conventional PDMS-based device) were randomly ordered across participants. Within each condition, lifts with the four different load forces were conducted in a randomized order, with each lift performed 10 times with the same load force.
To remove the effect of the interaction between the finger and the object on the grip force, a slip ratio experiment was carried out. After the object was lifted up, participants were asked to slowly release the force of the fingers until the object drops. The last grip force prior to the object slipping from the grasp was used to estimate the minimum level of the grip force for every load force ( Supplementary Fig. 15a). The median values over five trials for each participant are adopted. The relationship among the minimum grip force (Fg'), the load force (Fl), and the friction coefficient (μf) meets the following equation:

Supplementary Note 1: Formation process of wet heterostructured jets.
In this work, two kinds of wet heterostructured jets (i.e., the bead-on-string PVDF, TPU or PVA jets and the fibrous PVA jets with uneven diameter (Supplementary Fig. 3c and 12)) can spontaneously compose EMPAs. Their formation process is as follows. The joint effect of electrostatic force and surface tension drives the jet to eject from the Taylor cone forming at the spinneret tip 13 . When surface tension exerts a greater influence on jets than Coulombic repulsive force, electrified jets with nonuniform specific surface areas (i.e., heterostructured jets) are apt to form 7,14,15 . Herein, a series of selective dilute solutions with low viscosity are used to enhance the surface tension acting on jets 5 and to obtain heterostructured jets with appropriate size as well as composition. Meanwhile, various solvents (DMSO and water) with low saturated vapor pressure (i.e., high boiling point) are used to ensure incomplete volatilization of the solvent just as the jets come into contact with the collector. Therefore, the jet region with low specific surface area, such as the bead part, is wet.

Supplementary Note 2: Formation process of differently charged initial fibers.
The electrospinning of wet heterostructured electrified jets can lead to differently charged initial fibers deposited on a collector. The deposited microdomains containing less PVDF carry positive charges, but the deposited microdomains containing more PVDF are negatively charged 7,14 . The formation process of these differently charged initial fibers is shown in Supplementary Fig. 4. When the first batch of positively charged bead-on-string PVDF jets eject from the spinneret tip and crash against the negatively charged aluminum foil electrically grounded, positive charges will be remaining on the string parts for a time due to their high electrical insulation 16,17 . In contrast, it is hard for the initially deposited bead parts with low insulation to continue binding positive charges. Under the effect of the electric field in the electrospinning machine, the initially deposited bead parts are easy to be polarized 18,19 , and therefore negative charges appear on the top surface of bead parts (Supplementary Fig.   4c). Subsequently, the deposited microdomains enriched with positive charges repel the next batch of positively charged jets 20 . On the contrary, the aerial jets are attracted by the deposited microdomains with negative charges (Supplementary Fig. 4c), 19 composing new thicker negatively charged microdomains.
Additionally, the charged situation of the collector and the insulation difference between the string parts and the bead parts are further illustrated by the following two sections. Firstly, to determine the charged situation of the collector, we tested the surface potential of the aluminum foil (i.e., the collector). As shown in Supplementary Fig. 5a, when the voltage of the power supply in the electrospinning machine is zero, the surface potential of the aluminum foil is zero. When the voltage reaches up to +12.5 kV, the surface potential of the aluminum foil is -4.3 kV (Supplementary Fig. 5b). It is concluded that net negative charges appear on the surface of the aluminum foil under the effect of the electric field at the beginning of electrospinning.

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Secondly, the insulation difference was experimentally demonstrated. For ease of description, the string part and the bead part of the bead-on-string fiber are designated as "nanowire" and "microsphere", respectively. In the process of electrospinning, the electrified jets are ejected from the spinneret and then accelerated by the electric field. The aerial jets subjected to the electric field force are elongated and thinned 21  A weighing experiment can indicate that partial solvent still remains in initially deposited microspheres. Compared with the fresh EMPA films at the end of electrospinning, the EMPA films dried at room temperature for more than 6 h exhibited significant decrease in weight.
For example, the mass of the 3D EMPA-M films is reduced by approximately 70% after drying.
To verify the insulation difference between the initially deposited nanowires and microspheres, we tested the resistance of an electrospun PVDF film before and after wetting with the solvent DMSO. The experimental procedure is shown in Supplementary Fig. 6. An ultrathin dry PVDF film (thickness ~ 5 μm) was deposited on a smooth flat titanium (Ti) plate by electrospinning ( Supplementary Fig. 6a, b). Subsequently, another smooth flat Ti plate was placed on top of the electrospun-film-deposited Ti plate. With two Ti plates as electrodes, the resistance of the dry electrospun film was tested ( Supplementary Fig. 6c). When the centre of the electrospun film was wetted with DMSO, the film resistance was tested as well. The S43 resistances of the electrospun film before and after wetting treatment are 759077 and 0.1397 kΩ, respectively, which reflects the remarkable insulation difference.

Supplementary Note 3: Necessity of the wet heterostructured electrified jets for fabricating EMPAs.
By virtue of wet rather than dry heterostructured electrified jets, a variety of EMPAs are successfully fabricated (Fig. 1c and 2c). When we choose low-boiling-point solvents or raised temperature during electrospinning to make the deposited heterostructured jets dry, only flat electrospun films are prepared (Supplementary Fig. 13). Note that other experimental conditions remained unchanged. This result indicates the necessity of the wet heterostructured electrified jets.

Supplementary Note 4: The reason why negative charges appear on the tops of electrospun microdomes and micropyramids, and the reason why the local electric field is higher at a position closer to the tip of the microdome or micropyramid.
Electrostatic induction and polarization under the effect of the electric field in the electrospinning machine lead to the appearance of negative charges on the tops of electrospun micro-protuberances (i.e., microdomes and micropyramids) 19 . As shown in Supplementary   Fig. 7, the surface potential value of the deposited EMPA film during electrospinning is a minus, which indicates that the film surface facing the spinneret is macroscopically electronegative. Obviously, there must be microdomains that are negatively charged. SEM images and statistical data in Supplementary Fig. 8 show that the position closer to the top of a micro-protuberance has a higher fiber density, and the fibers densely accumulate at the tip of a micro-protuberance. Since positively charged aerial jets tend to deposit on the microdomains carrying more negative charges, it is concluded that there are negative charges at the tops of microdomes and micropyramids.
Finite element simulation was used to illustrate the local electric fields at different positions for the electrospun micro-protuberances. As shown in Supplementary Fig. 9, whether for an electrospun microdome or an electrospun micropyramid, the position closer to the tip of the micro-protuberance has a higher local electric field.

Supplementary Note 5: Dependence study between voltage and size of EMPAs.
We control the voltage to adjust the size, composition, and charge density of heterostructured jets. As the voltage increases from 10.0 to 17.5 kV, the micropyramid arrays first appear, then dwindle in size, and finally disappear. When the voltage is only 10.0 kV, both the quantity ratio of beads to springs and the charge density on jet surfaces are too low, failing to meet the self-assembly conditions ( Supplementary Fig. 10a-i). When the voltage is 12.5 kV, the fiber size and the quantity ratio of beads to springs reach the best. Typical EMPAs are successfully fabricated ( Supplementary Fig. 10a-ii). A higher voltage of 15.0 kV makes parameters involving the jets deviate from optimum values, causing the smaller micropyramid size (Supplementary Fig. 10a-iii). Once the voltage reaches up to 17.5 kV, oversized bead-like jets make the self-assembly impossible ( Supplementary Fig. 10a-iv).

Supplementary Note 6: Removal of the interaction between the finger and the object.
In the object-grasping experiment, surface material (P < 0.001), load force (P < 0.001) and interaction between the finger and the object (P < 0.001) are the main determinants of the grip force. To remove the effect of interaction on the grip force, we subtract the minimum grip force necessary (i.e., the last grip force, Supplementary Fig. 15a) based on the grasp friction coefficient (Supplementary Fig. 15b) and express the amount of grip force exceeding this level (i.e., additional grip force, Fig. 3h).

Supplementary Note 7: Purpose of the experiment about the EMPA films with different
APHs.
The precise structure-performance relationships of the micropyramid-arrayed devices in terms of daytime radiative cooling 23  carrying out a precise structure-performance relationship study.

Supplementary Note 8: Energy conversion efficiency of the improved TENG.
The energy conversion efficiency (η) of the nanogenerator is defined as the ratio of the output electrical energy transferred to the external load and the input mechanical energy 31 . The input mechanical energy consists of (i) the work done by a compression force between the initial contact state and the complete contact state as well as (ii) the work done by a force separating a pair of tribolayers.
A custom-made apparatus based on offset slider crank mechanism was first used to determine the mechanical-to-electrical energy conversion efficiency. When the angle between the horizon and the crank (α) is approximately −0.056π, a pair of tribolayers start touching each other (i.e., initial contact state, Supplementary Fig. 19a). When the α is approximately 0.067π, tribolayers are in full contact (i.e., complete contact state, Supplementary Fig. 19b).
The work done between the initial contact state and the complete contact state (Wi) can be expressed as: where Fc is the compression force, l is the moving distance of the slider. l1 in Supplementary   Fig. 19b is the moving distance of the plane between the initial contact state and the complete contact state. Supplementary Fig. 19d shows the Fc-l curve when a TENG consisting of a 3D EMPA-M and a flat electrospun PA66 film (i.e., the improved TENG) is sandwiched between the slider and the fixed block. The calculated W i is approximately 89 μJ. Supplementary Fig.   19e shows the output power curve of the improved TENG. The electrical energy of the TENG is completely released at 0.01 s after the complete contact state (i.e., separation state, Supplementary Fig. 19c). At this time the α is approximately 0.227π, and the distance between the separation state and the complete contact state (l2) is approximately 140 μm. The work done by a force separating a pair of tribolayers (Wii) can be expressed as: where Fs is the force that enables a pair of tribolayers to separate from each other.
where m is the mass of the tribolayer attached to the slider, and a is the acceleration of the tribolayer attached to the slider.
where ω is the angular velocity of the crank, r is the radius of the crank, L is the length of the rod, and e is the offset. l2 is put into Supplementary Eq. 3 to evaluate the required Wii as high as possible. The calculated W ii is less than 0.15 μJ, which is so small compared to W i that it can be negligible. Therefore, η can be evaluated according to the following equation: where U is the output voltage, R is the resistance of the best matched external load (1-50 MΩ), and t is the time.
Supplementary Fig. 19f shows the average values of the input mechanical energy and the output electrical energy of the improved TENG after replications of five sets of experiments.
The biomechanical-to-electrical energy conversion efficiency of the improved TENG is also estimated by using the same approach. Supplementary Fig. 20a shows the set-up of the measurement. The tribopositive and tribonegative layers are respectively attached to a force sensor and a board that is fixed on a hand. The force and electrical signals are recorded when the subject pats the force sensor ( Supplementary Fig. 20b). The calculated bioenergy harvesting efficiency is 41%.

Supplementary Note 9: Drawbacks of the commercial photoplethysmograph.
In terms of the commercial photoplethysmograph, a series of drawbacks stem from its hard as well as heavy structure and working mechanism of photoplethysmography. If the photoplethysmograph is placed so tightly on the person, ischemic pressure necrosis may occur. 3D EMPA-L (Fig. 1c, 2a-II, 2b-i, Supplementary Fig. 10a-ii, 10b- Severe discomfort (Exerting significant impact on work and daily life) 8 9 10 Extreme discomfort (Unable to engage in normal activities at work and in life) Under a solar intensity of 1 kW m -2 , the sample stage is covered with the fabric.

3.8°C below ambient
The skin covered with different sample films is exposed to sunlight with an intensity of 1 kW m -2 for 8 min.
4.0°C lower than the skin exposed to sunlight 2.4°C lower than the skin covered with a cottoncontaining fabric.
The cooling temperature values marked in orange represent the data measured under intense solar radiation (solar irradiance > 600 W m -2 ). The values marked in navy blue mean that the cooling temperatures are recorded without the influence of direct or intense sunlight.